Getmail is not a tool to migrate emails, as every email will get a new date. Getmail is used if you have mailboxes at several providers and want to fetch the emails to one mailbox every x minutes. Normally you migrate emails with a tool like imapsync or you copy the maildir files if the old server sues maildir too. Getmail connects to the mailbox with pop3 and empties the mailbox afterwards. If you did not test before that the target mailbox and the mailserver that is the target of the emails work, it might happen that the emails get lost. But most likely the emails are still in the mailqueue.

You can check in the mailqueue with the command:

postqueue -p

and you can also see in the mail log what happened with the emails and why they did not arrive in the target mailbox.